1. Field of the Invention
The present invention generally relates to an optical modulator and a method of manufacturing the optical modulator, and more particularly, to an optical modulator having modulating electrodes and a method of manufacturing such an optical modulator.
2. Description of the Related Art
Traveling wave electrodes are normally employed for an optical modulator that is required to operate at 10 Gbps or higher. Such an optical modulator with traveling wave electrodes has a special electrode structure to improve the modulation band. This structure is disclosed by L. Mori, D. Hoffmann, K. Matzen, C. Bornholdt, G. G. Mekonnen, and F. Reier, in “Traveling Wave Electrodes for 50 GHz Operation of Opto-Electronic Device Based on InP, (11th International Conference on Indium Phosphide and Related Materials, Davos, Switzerland, pp. 385-388, 16-20 May, 1999)”, or ELECTRONICS LETTERS 9th November 1989 Vol. 25, No. 23, pp. 1549-1550. FIGS. 1A and 1B show such a structure as Prior Art 1.
FIG. 1A is a top view of a Mach-Zehnder optical modulator. As shown in FIG. 1A, the Mach-Zehnder optical modulator has an n+-type GaAs layer 112 as a lower electrode layer on a semi-insulating GaAs (gallium arsenide) substrate 111. Further, an AlGaAs layer 114, a GaAs layer 115, and AlGaAs layers 105 and 106 are formed as an optical waveguide structure on the n+-type GaAs layer 112.
The GaAs layer 115 located at the center of the optical waveguide structure is the core layer. The AlGaAs layers 114 and 105 (106) that sandwich the core layer are undoped cladding layers.
This Mach-Zehnder optical modulator includes traveling wave electrodes 101 and 102 that mainly transmit modulating signals, and modulating electrodes 103 and 104 that transmit modulating signals directly to light to be modulated. This structure eliminates the problem that the transmission rate of modulating signals being transmitted through the traveling wave electrodes 101 and 102 is normally higher than the transmission rate of light (to be modulated) being transmitted through the core layer. Since several modulating electrodes 103 (104) are allocated to the traveling wave electrode 101 (102), the transmission rate of modulating signals being transmitted through the traveling wave electrodes 101 and 102 can be reduced to equal the transmission rate of light to be modulated. This can be done because the modulating electrodes 103 and 104 serve as capacitive loads on the traveling wave electrodes 101 and 102, respectively.
The traveling wave electrodes 101 and 102 are electrically separated from each other by grooves 107 and 108 that are formed by trenching and reach the substrate 111 under the embedded n+-type GaAs layer 112. Accordingly, the segmented modulating electrodes 103 and 104 are electrically connected to the traveling wave electrodes 101 and 102, respectively, with an air-bridge structure (117, 118).
With this particular structure, the transmission rate of modulating signals is equal to the transmission rate of light to be modulated in the Mach-Zehnder optical modulator shown in FIGS. 1A and 1B. Also, the characteristic impedance of the entire modulator is matched to a predetermined value that is normally 50 Ω.
In recent years, there has been an increasing demand for optical modulators with even higher performance. One of the highly demanded features is a lower operating voltage.
In Prior Art 1 shown in FIGS. 1A and 1B, however, there exist the undoped (or semi-insulating) cladding layers 105 and 106 between the core layer 115 and the modulating electrodes 103 and 104, respectively. Therefore, it is difficult to efficiently induce an electric field in the core layer 115 with modulating signals. To eliminate this difficulty, the cladding layers 105 and 106 are formed from conductive layers (this structure will be hereinafter referred to as Prior Art 2). In this structure, however, each neighboring modulating electrodes 103 (104) are connected to each other with the conductive cladding layer 105 (106), resulting in a large loss of a modulating signal and a difference in transmission rate between light to be modulated and modulating signals.